<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>监听属性</title>
    <script src="https://cdn.jsdelivr.net/npm/vue"></script>
</head>

<body>
<div id = "computed_props">
    千米 : <input type = "text" v-model = "kilometers">
    米 : <input type = "text" v-model = "meters">
</div>
<p id="info"></p>


<script type = "text/javascript">
    var vm = new Vue({
        el: '#computed_props',
        data: {
            kilometers : 0,
            meters:0
        },
        methods: {
        },
        computed :{
        },
        watch : {
            kilometers:function(val) {
                this.kilometers = val;
                this.meters = val * 1000;
            },
            meters : function (val) {
                this.kilometers = val/ 1000;
                this.meters = val;
            }
        }
    });


    // $watch 是一个实例方法
    vm.$watch('kilometers', function (newValue, oldValue) {
        // 这个回调将在 vm.kilometers 改变后调用
        document.getElementById ("info").innerHTML = "修改前值为: " + oldValue + "，修改后值为: " + newValue;
    })
</script>


</body>
</html>